This invention relates to endodontic instruments having non-circular cross sections, and more specifically, to endodontic instruments for removing pulp tissue and widening root canals that include helical cutting surfaces and helical non-contact surfaces for improving performance, decreasing stress, and removing debris during operation.
When a diseased root canal (diseased pulp tissue) is found in the root canal of a patient's tooth, a dentist removes the pulp tissue in order to get access to the most apical end of the tooth where bacteria is normally found. Usually a series of files or reamers (hand instruments or rotary instruments) which have a circular cross section are used to both remove the pulp tissue and to widen the root canal. Along with the tissue being removed mechanically, irrigation solutions like sodium hypochlorite (NaOCl) are used to kill any remaining bacteria and digest tissue remnants. Then the root canal is filled with gutta-percha and adhesive and sealed-off with such root canal preparations as the one sold by Essential Dental Systems, Inc. under the trademark EZ-Fill. Lastly, a crown could be fitted to the tooth.
The traditional endodontic instrument used to remove pulp tissue or dentin in the root canal has a circular cross section. As the instrument engages the dentin, it creates a contact surface extending 360 degrees about the narrowest part of the canal. This complete circumferential contact creates resistance to the rotation of the instrument as it removes dentin. The resistance is transferred to both the instrument and the tooth in the form of stress. The greater the resistance, the greater the stress. Because the tooth is much thicker than the reamer or file, it has a minimal chance of distorting or fracturing. The reamer or file, which can be hand or rotary driven however, can easily distort by unwinding when engagement along the shank produces excessive torque generated by either hand or motor powered rotation. In fact, if rotation with excessive torque continues, the stainless steel reamer can unwind and then fracture. Ni—Ti (Nickel-titanium) instrumentation will generally fracture much more abruptly given the same amount of torque because the Ni—Ti alloy is more flexible with a much lower elastic limit.
One way to reduce the chances of excessive engagement is by using a sequence of ever thickening instruments that remove the dentin in a gradual fashion. In theory, each succeeding reamer or file widens the canal enough to allow the incremental removal of dentin with the subsequent reamer or file never creating sufficient engagement for distortion during rotation. This technique is known as step-back because the wider reamers or files are also taken to a shallower depth to further minimize stress and distortion to the instrument.
Another technique employed in endodontics is called crown-down. It employs wider instruments first to open up the coronal aspects of the canal, and subsequently thinner reamers and files are placed more and more apically. Both techniques attempt to reduce the amount of torque generated by limiting the degree of dentin engagement that the reamers and files encounter, to prevent excessive engagement of dentin which leads to distortion and potential fracture of the instruments.
The implementation of these techniques with an endodontic instrument which does not make full engagement with the dentin in a root canal as taught, for example, in U.S. Pat. No. 6,042,376 entitled “Non-circular endodontic instruments” to Cohen, et al. (“the '376 Patent”), have resulted in the reduction of stress and the facilitation of debris removal from the root canal during endodontic instrumentation. FIG. 1 shows an embodiment of the '376 Patent in which an endodontic instrument 110 has an upper end 112 tapering along its length to a lower end 114, a cutting surface 120 including cutting blades 130 extending helically along cutting surface 120, and a non-contact area 140. Cutting surface 120 extends along the length of instrument 110 about a portion of the circumference of instrument 110. Non-contact area 140 also extends along the length of instrument 110 about a portion of the circumference of instrument 110.
In use, when the instrument of the '376 Patent is inserted into a canal and rotated, the cutting surface with the cutting blades engages the canal and removes dentin therefrom. The non-contact area does not engage the canal and thus reduces the stress that would be caused by the engagement of a completely circumferential cutting face with the canal. The non-contact area also provides an area for debris to gather and be maintained out of the way of the cutting face. However, when the instrument is used in reciprocation, there is a portion of the canal wall that remains unengaged, compromising the effectiveness of the canal cleansing.
There is a need for stronger endodontic instruments having improved configurations to resist fracturing and that also improve facilitation of debris removal from the root canal.